STELLAR “PROMINENCES”STELLAR “PROMINENCES”

• Mapping techniquesMapping techniques

• Mechanical supportMechanical support

• Short- and long-term Short- and long-term evolutionevolution

• Implications for coronal Implications for coronal structure and evolutionstructure and evolution

Andrew Collier CameronUniversity of St Andrews, Scotland.

Spots and prominences: signaturesSpots and prominences: signatures

• AB Dor, AB Dor, AAT/UCLES, AAT/UCLES, 1996 Dec 291996 Dec 29

• Donati et al 1998Donati et al 1998

Starspotsignatures inphotospheric lines

-v sin i +v sin i

Absorptiontransients inH alpha

-v sin i +v sin i

GoalsGoals• Neutral gas condenstions as probes of coronal structureNeutral gas condenstions as probes of coronal structure

– Radial distributionRadial distribution

– Inclination dependenceInclination dependence

• Determine physical properties of prominences at Determine physical properties of prominences at various distances from star.various distances from star.

• Measure timescales for Measure timescales for – Prominence formationdifferential rotationProminence formationdifferential rotation

– Surface Surface

– changes in coronal structurechanges in coronal structure

• Does flux emergence or surface differential rotation Does flux emergence or surface differential rotation drive coronal evolution?drive coronal evolution?

• Potential field extrapolations from magnetic mapsPotential field extrapolations from magnetic maps– Surface distribution of open field lines (coronal holes) Surface distribution of open field lines (coronal holes)

– Potential minima as prominence formation sites? Potential minima as prominence formation sites?

Radial accelerationsRadial accelerations• Radial acceleration of co-rotating cloud -> axial distanceRadial acceleration of co-rotating cloud -> axial distance

• Most transients have similar drift rates across HMost transients have similar drift rates across H profile profile

Axial distances of absorbing cloudsAxial distances of absorbing clouds• Clouds congregate mainly near or just outside Clouds congregate mainly near or just outside

co-rotation radius ( ). co-rotation radius ( ).

• AB Dor: Corotation radius is 2.7 R* from rotation axis.AB Dor: Corotation radius is 2.7 R* from rotation axis.

Coronal condensations: single starsCoronal condensations: single stars

• Detected in 90% of young (pre-) main Detected in 90% of young (pre-) main sequence stars with Psequence stars with Protrot<1 day<1 day

– AB Dor (K0V): AB Dor (K0V): Collier Cameron &Robinson Collier Cameron &Robinson 1989 1989

– HD 197890 =“Speedy Mic” (K0V): HD 197890 =“Speedy Mic” (K0V): Jeffries 1993Jeffries 1993

– 4 G dwarfs in 4 G dwarfs in Per cluster: Per cluster: Collier Cameron & Collier Cameron & Woods 1992Woods 1992

– HK Aqr = Gl 890 (M1V): HK Aqr = Gl 890 (M1V): Byrne, Eibe & Byrne, Eibe & Rolleston 1996Rolleston 1996

– RE J1816+541: RE J1816+541: Eibe 1998Eibe 1998

– PZ Tel: PZ Tel: Barnes et al 2000Barnes et al 2000 (right) (right) PProtrot = 1 day = 1 day (slowest yet)(slowest yet)

– Pre-main sequence G star RX J1508.6-4423 Pre-main sequence G star RX J1508.6-4423 (Donati et al 2000) (Donati et al 2000) --prominences in emission!--prominences in emission!

Physical properties:Physical properties:

• Areas: Areas: 3 x 103 x 102121 cm cm22 (up to 0.3 A(up to 0.3 A**))

• Column densities: Column densities: NNHH ~ 10 ~ 102020cmcm-2-2

• Masses: Masses: 2-6 x 102-6 x 101717 g g (cf solar quiescent (cf solar quiescent prominences M ~ 10prominences M ~ 101515 g) g)

• Temperatures: Temperatures: 8000-9000K8000-9000K

• Number: about Number: about 6-8 6-8 in observable slice of coronain observable slice of corona

• Co-rotation enforced out to about Co-rotation enforced out to about 8R8R* * in AB Dorin AB Dor

• AmbientAmbient coronal temperature coronal temperature T ~ 1.5 x10T ~ 1.5 x1077 K K

• (Physical data from simultaneous H(Physical data from simultaneous H + Ca IIK + Ca IIK absorption studies, absorption studies, Cameron et al 1990Cameron et al 1990))

Emission signaturesEmission signatures• Seen only in the most rapidly-rotating, early G Seen only in the most rapidly-rotating, early G

dwarfs, e.g. RX J1508.6 -4423 (Donati et al 2000):dwarfs, e.g. RX J1508.6 -4423 (Donati et al 2000):

Star is viewed at low inclination; uneclipsed H-emitting clouds trace out sinusoids

Tomographic back-projectionTomographic back-projection• Clouds congregate near co-rotation radius (dotted).Clouds congregate near co-rotation radius (dotted).

• Little evidence of material inside co-rotation radius.Little evidence of material inside co-rotation radius.

• Substantial evolution of gas distribution over 4 Substantial evolution of gas distribution over 4 nights.nights.

What’s holding them down?What’s holding them down?• Radial accelerations Radial accelerations ((22r sin i) r sin i)

show that most of the prominences show that most of the prominences lie at cylindrical radii near (but lie at cylindrical radii near (but some inside and and some some inside and and some substantially outside) the substantially outside) the equatorial equatorial co-rotation radiusco-rotation radius..

• Outside co-rotation radius, the Outside co-rotation radius, the gravitational force on the plasma gravitational force on the plasma isn’t enough to keep the clouds in a isn’t enough to keep the clouds in a synchronous orbit.synchronous orbit.

• So we need an extra inward force to So we need an extra inward force to keep them in co-rotation with the keep them in co-rotation with the star.star.

• Can use the Can use the magnetic tensionmagnetic tension of a of a closed magnetic loop to anchor the closed magnetic loop to anchor the cloud to the surface.cloud to the surface.

T

Condensations within equatorial Condensations within equatorial co-rotation radiusco-rotation radius

• Byrne, Eibe & Rolleston (1996) Byrne, Eibe & Rolleston (1996) found clouds found clouds substantially below co-rotation radius in single M1V substantially below co-rotation radius in single M1V rapid rotator HK Aqr. rapid rotator HK Aqr.

• Eibe (1998) Eibe (1998) mapped condensations in M1V rapid rotator mapped condensations in M1V rapid rotator RE J1816+541, also found clouds within corotation RE J1816+541, also found clouds within corotation radius.radius.

Latitude dependenceLatitude dependence• AB Dor prominences need to be anchored at high AB Dor prominences need to be anchored at high

latitude to cross stellar disk, since i = 60 degrees.latitude to cross stellar disk, since i = 60 degrees.

• What about other stars with different inclinations?What about other stars with different inclinations?– BD+22 4409: Low inclination, no transients found: BD+22 4409: Low inclination, no transients found: Jeffries et al Jeffries et al

19941994

High latitude downflows in High latitude downflows in BD+22 4409BD+22 4409

• Eibe, Byrne, Jeffries & Gunn Eibe, Byrne, Jeffries & Gunn (1999): (1999): No absorption transients No absorption transients seen in 2 nights of time-resolved seen in 2 nights of time-resolved echelle data from 1993 August.echelle data from 1993 August.

• Narrow emission profile: Narrow emission profile: FWHM(Ha) < FWHM(v sin i)FWHM(Ha) < FWHM(v sin i)

• Persistent red-shifted absorption Persistent red-shifted absorption at all phases at all phases

• Low inclination i~50Low inclination i~50oo

• Walter & Byrne (CS10 1998)Walter & Byrne (CS10 1998): : inflowing material in unsupported inflowing material in unsupported high latitude regions well within high latitude regions well within co-rotation surface?co-rotation surface? 1993 Aug 4 1993 Aug 5

Evolution of absorption transientsEvolution of absorption transients• Evolution of absorbing clouds around AB Doradus, Evolution of absorbing clouds around AB Doradus,

1996 December 23, 25, 27 & 29:1996 December 23, 25, 27 & 29:

AB Dor: starspot distribution AB Dor: starspot distribution 1996 Dec 23 - 291996 Dec 23 - 29

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AB Dor: Radial magnetic fieldAB Dor: Radial magnetic field 1996 Dec 23 - 29 1996 Dec 23 - 29

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Surface shear: AB Dor, 1996 Dec 23 - 29Surface shear: AB Dor, 1996 Dec 23 - 29

• CCF for CCF for surface-surface-brightness brightness images images

• CCF for CCF for magnetic magnetic images:images:

Donati et al (1998)

Phase drift of prominences Phase drift of prominences AB Dor 1995 Dec 7 to 11AB Dor 1995 Dec 7 to 11

Back projections sliced at 2.5 stellar radii

0.2 0.4 0.6 0.8 1.0 1.2 1.4Rotation phase

-0.030

-0.020

-0.010

0.000

0.010

0.020

Ab

BC

D

EAa

• Prominence Prominence rotation lags rotation lags equator.equator.

• Rotation rate Rotation rate matches surface at matches surface at latitude 60latitude 60oo to 70 to 70oo..

• cf. east-west cf. east-west alternating alternating magnetic polarity magnetic polarity pattern at same pattern at same latitude.latitude.

Donati & Cameron (1997)

Support in complex field structuresSupport in complex field structures

• Ferreira (1997): Ferreira (1997): component of effective gravity component of effective gravity along the field must be in stable balance.along the field must be in stable balance.

• Stable locations Stable locations exist inside corotation even for a exist inside corotation even for a dipole field (left) or quadrupole-sextupole (right)dipole field (left) or quadrupole-sextupole (right)

R K R K

Open field topology from ZDIOpen field topology from ZDI• AB Dor, 1995 December AB Dor, 1995 December

7-12.7-12.

• Zeeman Doppler image Zeeman Doppler image derived from echelle derived from echelle circular circular spectropolarimetry at spectropolarimetry at Anglo-Australian Anglo-Australian Telescope (Donati et al Telescope (Donati et al 1997)1997)

• Open field lines traced Open field lines traced from Zeeman-Doppler from Zeeman-Doppler image assuming potential image assuming potential field with source surface field with source surface at 5 stellar radii.at 5 stellar radii.

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Stable gravitational-centrifugal minimaStable gravitational-centrifugal minima• Potential-field models from Potential-field models from

Zeeman-Doppler images (ZDI) Zeeman-Doppler images (ZDI) show stable potential minima show stable potential minima along closed field lines along closed field lines satisfying:satisfying:

• Here gHere geffeff is the effective is the effective gravitational potential gradient gravitational potential gradient including centrifugal terms.including centrifugal terms.

• Condensations can be Condensations can be supported stably in these supported stably in these locations.locations.

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Image derived from AAT+UCLES+Semel polarimeter data, 1996 Dec 23+25

Jardine et al 2000, in preparation

geff.B=0 and

(B.∇)(geff .B)<0

Summary and conclusionsSummary and conclusions• Coronal condensations probe extent of closed-field Coronal condensations probe extent of closed-field

region in rapidly rotating late-type stars.region in rapidly rotating late-type stars.

• Prominences within corotation radius require Prominences within corotation radius require complex field topologies for support.complex field topologies for support.

• Can form up to 30Can form up to 30oo or so out of equatorial plane at or so out of equatorial plane at intermediate axial inclinations.intermediate axial inclinations.

• Downflows seen in BD+22 4409 suggest coronal Downflows seen in BD+22 4409 suggest coronal condensations form in unsupported regions too.condensations form in unsupported regions too.

• Prominence system evolves faster than surface Prominence system evolves faster than surface structure: coronal field continually destabilised by structure: coronal field continually destabilised by surface shear?surface shear?

• Where are the open field lines? Need to combine Where are the open field lines? Need to combine ZDI with prominence studies to obtain self-ZDI with prominence studies to obtain self-consistent picture of 3D coronal structure.consistent picture of 3D coronal structure.